This invention relates to heteroepitaxial layer formation in the fabrication of semiconductor devices and, more particularly, to such formation on a silicon carbide substrate surface.
Increasingly, laser diodes (LD) and light emitting diodes (LED) based on gallium nitride (GaN) thin films are being used because of their range of electrical, thermal, and optical properties. For a GaN-based laser diode to have a long life, i.e. several thousand hours of continuous operation, it is important to minimize the density of defects (usually in the form of dislocations) in the GaN film. This requires a defect density of less than about 1.times.10.sup.7 cm.sup.-2 (i.e. less than 10 million defects per square centimeter).
One attempt at achieving such a low defect density involves complex schemes of growing the GaN film epitaxially on a sapphire substrate. These growth schemes are time consuming, and their use impedes commercialization of devices based on GaN.
Another attempt is to grow the GaN thin film using a silicon carbide (SiC) substrate with a buffer layer of aluminum nitride (AlN) between the substrate and the GaN film. Such an attempt has potential because the GaN/AlN/SiC system has close matching of lattice parameters, namely 3.19/3.11/3.08 .ANG.. Prior heteroepitaxial structures of GaN and AlN on a SiC substrate have been fabricated as described in Torres et al., "Growth of AlN and GaN on 6H-SiC(0001) Using a Helium Supersonic Beam Seeded with Ammonia," Appl. Phys. Lett. 71 (10) (Sep. 8, 1997). In that process, the SiC substrate was first degreased, deoxidized, and degassed up to 500.degree. C. Then, the SiC substrate was annealed to 900.degree. C. for 20 minutes. Next, a buffer layer of AlN was grown on the SiC substrate at a temperature of between 560 and 1000.degree. C. Finally, a GaN layer approximately 0.1 .mu.m thick was grown on the AlN layer at a temperature exceeding 700.degree. C. In a typical structure where the AlN layer was grown at 900.degree. C. and the GaN layer was grown at 800.degree. C., defect densities of 2.times.10.sup.11 cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2 were measured for the AlN and GaN layers, respectively. Notwithstanding the reasonably good lattice matching of the GaN/AlN/SiC system, this process still produced relatively high defect densities.
Hallin et al., in "In situ preparation for Eligh-Quality SiC Chemical Vapour Deposition," J. Crystal Growth 181, pp. 241-53 (1997), have shown that etching a SiC substrate in hydrogen at 1500 to 1600.degree. C. improves the quality, with respect to defect formation, of an epitaxial layer grown on the substrate. However, a GaN thin film heteroepitaxial structure has still not been fabricated with low defect density.